Several trends have developed in lighting design to reduce the cost of interior lighting. The standard solution of brightly lighting an entire work environment has given way to more localized illumination of critical work areas with what may be termed task lights.
When used alone, ambient overhead lighting typically requires two to four watts per square foot for the efficient performance of most visual tasks. Task lights, on the other hand, serve to bring a concentrated and appropriate quality of light to specific areas in a manner that is conducive to comfort and productivity--and at half the energy cost. Combinations of task/ambient illumination reduces the wattage requirement below two watts per square foot.
Task lights are frequently used for lighting flat surfaces of either a horizontal or vertical orientation. Some examples of horizontal orientation are desks and tables, clerical and manufacturing work stations, and drawing boards. Typical vertical orientations include sheet music on stands and on pianos, and vertical document holders used in conjunction with Visual Display Terminals.
Horizontal work surfaces are frequently lit with incandescent or fluorescent task light sources positioned a foot or more above said surface. To provide sufficient light from that distance the lamp must be a relatively high wattage (e.g. 75 to 150 watts), with a substantial portion of the light spilling beyond the intended area of illumination. Additionally, a light source so directed will cause a significant amount of reflective glare.
The position of a task light is very critical to achieve acceptable uniformity of illumination over the task surface while also eliminating glare. A light placed directly over or in front of a task will provide uniform illumination but reflections in the surface will produce glare. This can be visualized by replacing the task with a mirror and if the person viewing the task sees the light in the mirror then reflected glare will occur. To address this issue, the task light must be placed well above, below or to the sides of the task to eliminate any direct reflection in the task surface. This oblique angle of light to the task and the close proximity of one side of the task to the light will result in non-uniform illumination. Providing light from two sides will produce acceptable uniformity and their placement can be chosen to eliminate reflected glare. Two units, however, increase cost and wattage while limiting versatility of the system performance under some user conditions.
The color quality of the illumination provided by the task light is an important consideration. The incandescent lamp and recently the tungsten halogen version of the incandescent lamp have been well received as the source used in task lights. These lamps provide a warm full color rendition that meets user quality concerns for task lights. In the past, attempts to replace the incandescent lamp with the more energy efficient fluorescent lamp have not been well received due to lamps being cooler and providing poor color rendition. Recent developments of tri phosphor fluorescent lamps have reversed this condition by making warm color fluorescent lamps that have a high color rendition as an available option. Such lamps can now be used in task lights to provide a more efficient means of supplying task illumination.
Piano sheet music is commonly lit with a desk-type lamp from above the music to minimize glare at the expense of good light distribution. There may be a drastic fall-off of light levels from the top to the bottom of the sheet music. Such a fall-off may be in the the range of from four to six:one. As a result, the music is difficult to read. Alternately, a floor lamp is sometimes placed behind the player for more uniform distribution, but with the angle of illumination resulting in uncomfortable levels of reflective glare.
The traditional light sources mentioned above possess a common limitation--they are incapable of providing the relatively even, glare-free lighting so desired.
One proposed solution to the piano music lighting problems was to use an aperture fluorescent lamp to light the music. In this arrangement, an extremely long lamp--required to be at least twice as long as the maximum distance from the lamp to the far side of the area to be lit--was mounted below the sheet music and the light-emitting aperture directed up onto the music. With a lamp at least twice as long as the height of the sheet music, or about thirty-six inches, the aperture lamp would theoretically provide sufficient light over the whole surface of the music. However, the extreme length of the lamp made it unsightly and difficult to mount below the music, much less over the music. In addition, the brightness of the long slot-shaped light emitting aperture caused problems with glare. As a result, the arrangement did not prove successful.